The present invention relates to a vehicle speed control apparatus of an industrial vehicle.
Japanese Patent Application Publication No. 7-11987 discloses a control apparatus of a forklift truck having a structure wherein the accelerator pedal is not connected to a throttle valve of an engine of the forklift truck and the control apparatus is configured to control the traveling speed of the forklift truck for an optimum fuel consumption, as well as to provide ordinary speed. Specifically, for achieving a target engine speed for low fuel consumption rate according to a throttle opening of an engine, the control apparatus is configured to control the throttle opening and transmission ratio of HST (Hydraulic Static Transmission) by feedback control based on vehicle speed and engine speed.
The control apparatus of the above-cited publication adopts feedback control for vehicle speed control. In the vehicle speed control by feedback control, engine output power is determined based on the deviation between a target vehicle speed and an actual vehicle speed. The deviation of the vehicle speed is decreased with a decrease of the target vehicle speed. For example, when the lift of a forklift truck is being raised by inching operating of the pedal, the forklift truck may fail to increase the engine speed to the desired level. Especially, in a forklift truck, in which the engine supplies power not only for traveling, but also for load handling unlike a passenger car, the engine speed needs to be increased to a level that is enough for the engine to supply required power for load handling, as well as for traveling, irrespective of a target vehicle speed. That is, the feedback gain of the vehicle control apparatus needs to be large. This is also true for a traction vehicle. In this case, the required power is depending on the presence or absence of any object to be towed and the load of the object.
In contrast, in the case of starting a vehicle wherein the deviation between a target vehicle speed and an actual vehicle speed is large, the target engine speed becomes too high if feedback gain is large, so that the vehicle may be accelerated excessively or overshoot in the vehicle speed may occur.
In the case of a gasoline engine whose response to a command for increasing the engine speed is slow, time lag occurs in increasing the engine speed and the deviation for feedback control is accumulated, so that the target engine speed becomes too large and overshoot in the vehicle speed tends to occur. In the case of a diesel engine whose response to the command is faster than in the gasoline engine, the vehicle tends to be accelerated excessively immediately after starting a vehicle and when the deviation of the vehicle speed is large.
The present invention which has been made in light of the problems described above is directed to providing a vehicle speed control apparatus of an industrial vehicle which prevents excessive acceleration of a vehicle and overshoot of a vehicle speed while securing an engine speed required for load handling.